1. Field of the Invention
The present invention relates to binding proteins and antigen-binding fragments thereof that bind interleukin-21 receptor (IL-21R), in particular, human IL-21R, and their use in regulating IL-21R-associated activities. The binding proteins disclosed herein are useful in treating and/or diagnosing IL-21R-associated disorders, e.g., inflammatory disorders, autoimmune diseases, allergies, transplant rejection, hyperproliferative disorders of the blood, and other immune system disorders.
2. Related Background Art
Antigens initiate immune responses and activate the two largest populations of lymphocytes: T cells and B cells. After encountering antigen, T cells proliferate and differentiate into effector cells, while B cells proliferate and differentiate into antibody-secreting plasma cells. These effector cells secrete and/or respond to cytokines, which are small proteins (less than about 30 kDa) secreted by lymphocytes and other cell types.
Human IL-21 is a cytokine that shows sequence homology to IL-2, IL-4 and IL-15 (Parrish-Novak et al. (2000) Nature 408:57-63). Despite low sequence homology among interleukin cytokines, cytokines share a common fold into a “four-helix-bundle” structure that is representative of the family. Most cytokines bind either class I or class II cytokine receptors. Class II cytokine receptors include the receptors for IL-10 and the interferons, whereas class I cytokine receptors include the receptors for IL-2 through IL-7, IL-9, IL-11, IL-12, IL-13, and IL-15, as well as hematopoietic growth factors, leptin, and growth hormone (Cosman (1993) Cytokine 5:95-106).
Human IL-21R is a class I cytokine receptor. The nucleotide and amino acid sequences encoding human IL-21 and its receptor (IL-21R) are described in International Application Publication Nos. WO 00/053761 and WO 01/085792; Parrish-Novak et al. (2000) supra; and Ozaki et al. (2000) Proc. Natl. Acad. Sci. USA 97:11439-44. IL-21R has the highest sequence homology to the IL-2 receptor β chain and the IL-4 receptor α chain (Ozaki et al. (2000) supra). Upon ligand binding, IL-21R associates with the common gamma cytokine receptor chain (γc) that is shared by receptor complexes for IL-2, IL-3, IL-4, IL-7, IL-9, IL-13 and IL-15 (Ozaki et al. (2000) supra; Asao et al. (2001) J. Immunol. 167:1-5).
IL-21R is expressed in lymphoid tissues, particularly on T cells, B cells, natural killer (NK) cells, dendritic cells (DC) and macrophages (Parrish-Novak et al. (2000) supra), which allows these cells to respond to IL-21 (Leonard and Spolski (2005) Nat. Rev. Immunol. 5:688-98). The widespread lymphoid distribution of IL-21R indicates that IL-21 plays an important role in immune regulation. In vitro studies have shown that IL-21 significantly modulates the function of B cells, CD4+ and CD8+ T cells, and NK cells (Parrish-Novak et al. (2000) supra; Kasaian et al. (2002) Immunity 16:559-69). Recent evidence suggests that IL-21-mediated signaling can have antitumor activity (Sivakumar et al. (2004) Immunology 112:177-82), and that IL-21 can prevent antigen-induced asthma in mice (Shang et al. (2006) Cell. Immunol. 241:66-74).
In autoimmunity, disruption of the IL-21 gene and injection of recombinant IL-21 have been shown to modulate the progression of experimental autoimmune myasthenia gravis (EAMG) and experimental autoimmune encephalomyelitis (EAE), respectively (King et al. (2004) Cell 117:265-77; Ozaki et al. (2004) J. Immunol. 173:5361-71; Vollmer et al. (2005) J. Immunol. 174:2696-2701; Liu et al. (2006) J. Immunol. 176:5247-54). In these experimental systems, it has been suggested that the manipulation of IL-21-mediated signaling directly altered the function of CD8+ cells, B cells, T helper cells, and NK cells.